Research Summary - 2

Average daily gains and response to artificial insemination in beef replacement heifers with persistent subclinical Theileria orientalis Ikeda genotype infection.

Date/Time: 9/25/2020    09:30
Author: Sierra R Guynn
Clinic: VMCVM
City, State, ZIP: BLACKSBURG, VA  24061

S.R. Guynn, DVM, PhD 1 ; K.K. Lahmers, DVM, PhD, DACVP 2 ; J.F. Currin, DVM, DABVP 1 ; S.M. Todd, MS 2 ; A.A. Carbonello, MS 3 ; L.L. Hungerford, DVM, MPH, PhD, CPH 4 ;
1Department of Large Animal Clinical Sciences, Virginia Maryland College of Veterinary Medicine, Blacksburg, VA 24061
2Department of Biomedical Sciences and Pathobiology, Virginia Maryland College of Veterinary Medicine, Blacksburg, VA 24061
3Virginia Tech Animal Laboratory Services, Virginia Maryland College of Veterinary Medicine, Blacksburg, VA 24061
4Department of Population Health Sciences, Virginia Maryland College of Veterinary Medicine, Blacksburg, VA 24061

Introduction:

Theileria orientalis Ikeda genotype is a tick borne hemoprotozoan that typically causes economic losses in dairy and beef cattle in Australia, New Zealand and Japan. Clinical signs of theileriosis from T. orientalis Ikeda include anemia, icterus, ill-thrift, and death. The acute phase of the infection has been associated with a decrease in milk production in dairy cows, decreased libido in dairy bulls, decreased live weight gain in beef bulls and increased mortality in naïve adults and calves. A sequela to acute infections within a herd is persistent subclinical infections and these have been associated with negative effects on reproduction in dairy cows and decreased mean daily gain in suckling beef calves. Prior to 2017, non-pathogenic genotypes of T. orientalis were present in the United States. In late 2017, T. orientalis Ikeda/genotype 2 was detected in beef cattle from multiple counties in Virginia and was associated with anemia, weakness and death. As of 2020, T. orientalis Ikeda has been identified in beef cattle in 22/95 Virginia counties and 4/55 West Virginia counties. Beef production through primarily cow/calf operations is the second largest agricultural commodity in Virginia. After detection of T. orientalis Ikeda in a subpopulation of spring breeding beef heifers at a replacement breeding farm, we thoroughly investigated the fall breeding heifers. The objective of this study was to determine if persistent subclinical infection with T. orientalis Ikeda affected the average daily gain from arriving on farm to pregnancy verification or conception rates to artificial insemination (AI) in beef replacement heifers.

Materials and Methods:

The fall heifers (174) arrived at the breeding farm on June 1, 2019, four weeks after weaning on their home farms. The heifers were weighed three times between arrival and breeding on 7/16/19, 9/26/19 and 11/6/19. A whole blood sample was collected from the tail vein on 10/23/19 for T. orientalis Ikeda testing. For the T. orientalis Ikeda testing, DNA was extracted from the whole blood sample for quantitative polymerase chain reaction (qPCR) analysis for T. orientalis Ikeda. Cycle count thresholds of <39 indicated a positive for T. orientalis, further qPCR tests indicated specificity to the T. orientalis Ikeda genotype. The heifers were synchronized with a 14 day CIDR® -PG and timed AI on 11/25/19. A week later, the heifers were exposed to bulls. On 2/4/2020, ultrasound diagnosis of pregnancy was performed and the heifers were weighed again. Average daily gain (ADG) was compared using a two-tailed Student’s T-test with equal variances as determined by the F-test. Pregnancy status results were compared with a Chi-square contingency table with Fisher’s exact test for significance.

Results:

Of the 174 heifers tested for T. orientalis Ikeda on 10/23/19, 13 heifers were positive. For determination of ADG, 11 heifers were removed (all negative heifers) due to not recording all pertinent weights. There is no significant difference (p=.77) between the ADG’s of the negative (1.60±.30 lbs/day) and positive (1.63±.25 lbs/day) heifers from farm arrival to pregnancy diagnosis. If ADG is calculated for each period between weights, there is no difference in ADG between the T. orientalis Ikeda negative and positive heifers at any period. For comparison of pregnancy status, three negative heifers died prior to pregnancy diagnosis therefore only 171 heifers are included in this analysis. Comparing the pregnancy status of T. orientalis Ikeda negative and positive heifers at pregnancy diagnosis, indicates no significant difference between AI bred or bull bred/open heifers (p=0.15).

Significance:

Though many facets of T. orientalis Ikeda infection in the US have not yet been determined, this research provides preliminary evidence that persistent subclinical infection in heifers does not affect ADG post weaning or reproductive capabilities in regards to timed AI.